U.S. patent number 6,476,409 [Application Number 09/553,857] was granted by the patent office on 2002-11-05 for nano-structures, process for preparing nano-structures and devices.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Tohru Den, Tatsuya Iwasaki.
United States Patent |
6,476,409 |
Iwasaki , et al. |
November 5, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Nano-structures, process for preparing nano-structures and
devices
Abstract
The present invention provides a nano-structure which can be
applied to various high-function devices. The nano-structure
includes an anodically oxidized layer having a plurality of kinds
of pores.
Inventors: |
Iwasaki; Tatsuya (Machida,
JP), Den; Tohru (Funabashi, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
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Family
ID: |
26456902 |
Appl.
No.: |
09/553,857 |
Filed: |
April 21, 2000 |
Foreign Application Priority Data
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Apr 27, 1999 [JP] |
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11-119102 |
Mar 30, 2000 [JP] |
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2000-093129 |
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Current U.S.
Class: |
257/13; 257/14;
257/E33.004; G9B/5.289; G9B/5.306 |
Current CPC
Class: |
B82Y
10/00 (20130101); B82Y 20/00 (20130101); B82Y
25/00 (20130101); C25D 11/16 (20130101); G02B
6/1225 (20130101); G11B 5/74 (20130101); G11B
5/743 (20130101); G11B 5/855 (20130101); H01F
1/0081 (20130101); H01F 10/007 (20130101); C25D
11/045 (20130101); G11B 2005/0005 (20130101); H01F
10/324 (20130101); H01F 41/30 (20130101); H01L
33/18 (20130101) |
Current International
Class: |
C25D
11/16 (20060101); C25D 11/04 (20060101); G11B
5/855 (20060101); H01F 10/00 (20060101); H01F
1/00 (20060101); G11B 5/74 (20060101); G11B
5/00 (20060101); H01F 10/32 (20060101); H01L
33/00 (20060101); H01L 029/06 (); H01L
031/032 () |
Field of
Search: |
;257/13-14 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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931859 |
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Jul 1999 |
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EP |
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8-246190 |
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Sep 1996 |
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JP |
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10-121292 |
|
May 1998 |
|
JP |
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98/09005 |
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Mar 1998 |
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WO |
|
Other References
JD. Joannopoulos, et al., Photonic Crystals, Princeton University
Press, Princeton, New Jersey, pp. 54-77 and 94-104 (1995). .
R.C. Furneaux, et al., "The formation of controlled-porosity
membranes from anodically oxidized aluminum", Nature, vol. 337, pp.
147-149 (1989). .
H. Masuda, Solid State Physics, vol. 31, No. 5, pp. 493-499 (1996).
.
H. Masuda, et al., "Fabrication of Gold Nanodot Array Using Anodic
Porous Alumina as an Evaporation Mask", Jpn. J. Appl. Phys., vol.
35, part 2, No. 1B, pp. L126-L129 (1996)..
|
Primary Examiner: Meier; Stephen D.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
What is claimed is:
1. A nano-structure comprising an anodically oxidized layer,
wherein the anodically oxidized layer comprises a plurality of
regions respectively containing pores of different diameters.
2. A nano-structure according to claim 1, wherein the plurality of
kinds of pores are respectively arranged at controlled positions in
the anodically oxidized layer.
3. A nano-structure according to claim 2, wherein the pores are
regularly arranged at constant intervals.
4. A nano-structure according to claim 3, wherein the pores are
arranged in a triangular lattice form.
5. A nano-structure according to claim 1, wherein at least one kind
of the plurality of kinds of pores are arranged in a line.
6. A nano-structure according to claim 1, wherein the anodically
oxidized layer is an anodically oxidized aluminum layer.
7. A nano-structure according to claim 6, wherein at least one of
the pores is filled with a filler.
8. A nano-structure according to claim 7, wherein the filler
comprises a dielectric material having a refractive index different
from that of the anodically oxidized aluminum layer.
9. A nano-structure according to claim 7, wherein the filler
comprises a semiconductor.
10. A nano-structure according to claim 7, wherein the filler has a
luminescent function.
11. A nano-structure according to claim 7, wherein the filler
comprises a magnetic material.
12. A nano-structure according to claim 7, further comprising an
electrode in contact with the bottom of each of the pores so that
the electrode is electrically connected to the filler.
13. A nano-structure comprising an anodically oxidized layer
containing a first pore and a second pore, wherein the diameter of
the first pore is different from that of the second pore, and the
first and second pores are provided at controlled positions in the
layer respectively.
14. A nano-structure according to claim 13, wherein the anodically
oxidized layer is an anodically oxidized aluminum layer.
15. A nano-structure according to claim 13 or 14, wherein at least
one of the first and second pores is filled with a filler.
16. A nano-structure according to claim 15, wherein the filler
comprises an insulator.
17. A nano-structure according to claim 15, wherein the filler
comprises a semiconductor.
18. A nano-structure according to claim 15, wherein the filler has
a luminescent function.
19. A nano-structure according to claim 15, wherein the filler
comprises a material having a refractive index different from that
of the anodically oxidized aluminum layer.
20. A nano-structure according to claim 15, wherein the filler
comprises a magnetic material.
21. A light emitting device comprising a nano-structure having
pores according to claim 1 or 13, wherein the pores are filled with
a material having a luminescent function.
22. An optical device comprising a nano-structure having pores
according to claim 1 or 13, wherein the pores are filled with a
material having a refractive index different from that of the
anodically oxidized aluminum layer.
23. A magnetic device comprising a nano-structure having pores
according to claim 1 or 13, wherein the pores are filled with a
magnetic material.
24. A magnetic device according to claim 23, wherein the pores are
filled with a lamination of a ferromagnetic material and a
nonmagnetic material.
25. A nano-structure comprising an oxidized layer, wherein the
oxidized layer comprises a plurality of regions respectively
containing pores of different diameters.
26. A nano-structure according to claim 25, wherein the oxidized
layer is an aluminum oxide layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nano-structures and devices using
the same, and a process for preparing the nano-structures.
Particularly, the present invention relates to nano-structures
having pores, which is believed to be widely used as, for example,
electronic and optical devices, functional materials for
micro-devices, structural materials, etc., devices using the
nano-structures, and a process for preparing the
nano-structures.
2. Description of the Related Art
Some thin films, wires and dots of metals or semiconductors, which
have sizes smaller than a certain length, exhibit specific
electrical, optical and chemical properties due to enclosure of
electron movement. From this viewpoint, materials (referred to as
"nano-structures" hereinafter) having a fine structure of several
100 nm or less have increasingly attracted attention as functional
materials.
An example of processes for preparing such nano-structures
comprises preparing a nano-structure directly by a semiconductor
processing technique such as a patterning technique such as
photolithography, electron beam exposure, X-ray exposure, or the
like.
Besides this preparing process, an attempt is made to realize a
novel nano-structure comprising a regular structure naturally
formed, i.e., a structure formed in a self-ordering manner, as a
base. This process can possibly produce a specific fine structure
superior to structures produced by conventional processes depending
upon the fine structure used as the base, and thus many studies
have been conducted.
An example of such a self-ordering process is anodic oxidation
which can easily produce a nano-structure having pores in nano-size
with high controllability. For example, anodic porous alumina
formed by anodically oxidizing aluminum or an alloy thereof in an
acidic bath is known.
Anodic oxidation of an Al plate in an acidic electrolyte forms a
porous oxide film (anodic porous alumina) (refer to, for example,
R. C. Furneaux, W. R. Rigby & A. P. Davids on NATURE Vol. 337,
P147 (1989)). The porous oxide film is characterized by having a
specific geometric structure in which very fine cylindrical pores
(nano-holes) having a diameter of several nm to several hundreds nm
are arranged in parallel at intervals (cell size) of several nm to
several hundreds nm. The cylindrical pores have a high aspect
ratio, and are excellent in uniformity of the sectional diameter.
The diameter and interval of the pores can be controlled to some
extent by controlling the current and voltage in anodic oxidation,
and the thickness of the oxide film and the depth of the pores can
be controlled to some extent by controlling the anodic oxidation
time.
In order to improve the perpendicularity, linearity, and
independence of the pores of the anodic porous alumina, a two-step
anodic oxidation process has been proposed, in which after a porous
oxide film formed by anodic oxidation is removed, anodic oxidation
is again performed to form anodic porous alumina (ordered alumina
nanohole) having pores having good perpendicularity, linearity and
independence (Jpn. Journal of Applied Physics, Vol. 35, Part 2, No.
1B, pp. L126-L129, Jan. 15, 1996). This process utilizes the
property that surface concaves of an aluminum plate formed by
removing the anodic oxide film formed by first anodic oxidation
serve as the starting points of pore formation in second anodic
oxidation.
Besides these processes, the process of forming pore starting
points by using press pattering has also be proposed, in which a
substrate having a surface comprising a plurality of convexes is
pressed on the surface of an aluminum plate to form concaves as
pore starting points, and then anodic oxidation is performed to
form a porous oxide film having pores exhibiting good shapes,
interval and pattern controllability (Japanese Patent Laid-Open No.
10-121292).
In consideration of the specific geometric structure of the anodic
porous alumina, various applications are attempted. Although this
is explained in detail by Masuda, examples of application are
described below. Examples of applications include applications to
films using the anodically oxidized film having abrasion resistance
and insulation resistance, applications to filters using separated
films, etc. Furthermore, various other applications to coloring,
magnetic recording media, EL light emitting devices,
electro-chromic devices, optical devices, gas sensors, etc., are
attempted by using the technique of filling nano-holes with a
metal, a semiconductor, or the like, and the technique of
replicating the nano-hole structures. Furthermore, applications to
various fields of quantum fine wires, quantum effect devices such
as a MIM device, a molecular sensor using nano-holes as chemical
reaction fields, etc. are expected (Masuda, Solid State Physics,
31, 493 (1996)).
Since the above-mentioned direct preparation of nano-structures by
the semiconductor processing techniques has the problems of low
yield and high equipment cost, a simple preparation process having
high reproducibility is demanded.
From this viewpoint, the self-ordering process, particularly the
anodic oxidation process, is preferred because it can easily
prepare nano-structures with high controllability, and prepare
nano-structures in a large area. Particularly, the structure of
anodic porous alumina formed by two-step anodic oxidation or press
patterning, in which pores are regularly arranged, are preferred
from the viewpoint of structural uniformity of perpendicularity,
linearity, and arrangement of the pores.
SUMMARY OF THE INVENTION
In the process of studying applications of nano-structures to
devices, the inventors confirmed that an arrangement of two kinds
of pores having different diameters at controlled positions in a
nano-structure permits expansion of the range of applications of
nano-structures to devices. For example, it is expected that a
material having a structure in which the dielectric constant
(refractive index) periodically changes in a cycle of length near
the wavelength of light produces photonic crystals, thereby
permitting a high degree of light control. More specifically, a
photonic band gap in which the presence of light is inhibited in a
predetermined wavelength range is formed, or light is localized in
a predetermined wavelength range to enable applications of
non-structures to a light guide, a light emitting device, etc. One
of the two kinds of pores having different diameters can be
possibly used as photonic band gap regions, or regions where light
is localized. In addition, in filling pores having different
diameters with a magnetic material, the strength of a magnetic
field required for reversing the magnetization direction possibly
changes with changes in diameter of the pores. This can be possibly
applied to, for example, formation of tracking tracks on a
recording medium.
An example of conventional known methods of controlling the
diameters of the pores of anodic porous alumina is to immerse
alumina in an acidic solution (pore widening). However, this method
basically controls the pores to the same diameter, and cannot
control independently the diameters of the pores.
As a result of repetition of various studies in consideration of
the above-described technical background, the inventors found a
method for forming a nano-structure in which at least two kinds of
pores having different diameters are respectively arranged at
controlled positions, leading to the achievement of the present
invention.
Accordingly, an object of the present invention is to provide a
nano-structure having a construction for widening the range of
applications to various devices, and a light emitting device, an
optical device and a magnetic device using the same.
Another object of the present invention is to provide a process for
preparing a nano-structure having a novel construction for widening
the range of application to devices having a novel structure.
In accordance with a first aspect of the present invention, there
is provided a nano-structure comprising an anodically oxidized
layer, wherein the anodically oxidized layer comprises a plurality
of kinds of pores.
In accordance with another aspect of the present invention, there
is provided a nano-structure comprising an anodically oxidized
layer containing a first pore and a second pore, wherein the
diameter of the first pore is different from that of the second
pore, and the first and second pores are respectively provided at
controlled positions in the layer.
In accordance with still another aspect of the present invention,
there is provided a light emitting device comprising a
nano-structure comprising an anodically oxidized layer having a
plurality of kinds of pores, wherein the pores are filled with a
material having a luminescent function.
In accordance with a further aspect of the present invention, there
is provided a light emitting device comprising a nano-structure
comprising an anodically oxidized layer containing a first pore and
a second pore having different diameters, wherein the first and
second pores are respectively provided at controlled positions in
the anodically oxidized layer, and at least one of the first and
second pores is filled with a material having a luminescent
function.
In accordance with a further aspect of the present invention, there
is provided a light emitting device comprising a nano-structure
comprising an anodically oxidized layer having a plurality of kinds
of pores, wherein the pores are filled with a material having a
refractive index different from that of the anodically oxidized
layer.
In accordance with a further aspect of the present invention, there
is provided a light emitting device comprising a nano-structure
comprising an anodically oxidized layer containing a first pore and
a second pore having different diameters, wherein the first and
second pores are respectively provided at controlled positions in
the anodically oxidized layer, and at least one of the first and
second pores is filled with a material having a refractive index
different from that of the anodically oxidized layer.
In accordance with a further aspect of the present invention, there
is provided a magnetic device comprising a nano-structure
comprising an anodically oxidized layer having a plurality of kinds
of pores, wherein the pores are filled with a magnetic
material.
In accordance with a further aspect of the present invention, there
is provided a light emitting device comprising a nano-structure
comprising an anodically oxidized layer containing a first pore and
a second pore having different diameters, wherein the first and
second pores are respectively provided at controlled positions in
the anodically oxidized layer, and at least one of the first and
second pores is filled with a magnetic material.
In accordance with a further aspect of the present invention, there
is provided a process for preparing a nano-structure comprising an
anodically oxidized layer having a plurality of kinds of pores, the
process comprising the steps of preparing a film containing
aluminum and having a plurality of kinds of starting points for the
respective pores on a surface thereof, and anodically oxidizing the
film containing aluminum, wherein the plurality of kinds of staring
points are different in at least one of shape and composition.
In accordance with a further aspect of the present invention, there
is provided a process for preparing a nano-structure comprising an
anodically oxidized layer having first and second pores having
different diameters, the process comprising the steps of preparing
a film containing aluminum and having first and second starting
points for the respective pores on the surface thereof, and
anodically oxidizing the surface, wherein the first and second
starting points are different in at least one of shape and
composition.
The nano-structure having the above construction is formed by
forming pore starting points at desired positions in a workpiece,
and then anodically oxidizing the workpiece. In forming the pore
starting points, the shape or composition of each of the pore
starting points is controlled to independently control the
diameters of the respective pores of anodic porous alumina. This
method can realize a porous material having pores which have
desired diameters and are regularly arranged at desired
positions.
In the nano-structure of the present invention, the pores may be
filled with a functional material such as a metal, a semiconductor,
or the like to cause the possibility of application to new
electronic devices.
The nano-structure of the present invention can also be used as a
mold or mask to form a new nano-structure. For example, a porous
material having through pores, which is obtained by removing
portions of the nano-structure of the present invention other than
the porous portion, can be used as a mask for deposing a functional
material such as a metal, a semiconductor, or the like, or provided
as an etching mask on another substrate, to form a nano-structure
for quantum dots, or the like.
The nano-structure of the present invention can be used for various
applications such as a quantum wire, a MIM element, a molecular
sensor, coloring, a magnetic recording medium, an EL light emitting
device, an electro-chromic device, an optical device such as a
photonic crystal, an electron emitting device, a solar cell, a gas
sensor, an abrasion resistant-insulating resistant film, a filter,
etc. The nano-structure has the function to widen the range of
application thereof.
Particularly, a material having a structure in which the dielectric
constant periodically changes in a cycle of a length near the
wavelength of light forms photonic crystals, and has the
possibility of enabling a high degree of light control. More
effectively, a photonic band gap appears, in which the presence of
light is inhibited in a predetermined wavelength range (Photonic
Crtstals, J. D. Joannopoulos, R. D. Meade, and J. N. Winn,
Princeton University Press). Anodic porous alumina having a regular
arrangement of pores can be used as a photonic crystal by utilizing
the periodic structure thereof. In the present invention, the
technique of independently controlling the diameters of pores of
anodic porous alumina having regularly arranged pores permits
control of the structure of a photonic crystal, control of the
structure of a photonic band, and the formation of a waveguide or
defects. In the photonic crystal, a localized state of light can be
obtained by introducing defects, and thus a localized state of
light can be obtained by locally changing the diameters of some of
the pores of anodic porous alumina. This permits further
application to optical recording media, and the like.
With a photonic crystal comprising a luminescent material arranged
therein, a photonic band is appropriately designed according to the
emission wavelength to permit control of spontaneous emission, and
an improvement in performance of a light emitting device can thus
be expected. Namely, the pores of the above-described anodic porous
alumina are filled with a luminescent material to make it possible
to expect the realization of a light emitting device with a low
threshold value, a light emitting device with a narrow emission
spectral width, a laser with a low threshold value, etc.
Furthermore, the pores of the anodic porous alumina are filled with
a magnetic material to obtain magnetic nano-wires, and the pores of
the anodic porous alumina, which have different diameters, are
filled with a magnetic material to form an arrangement of magnetic
fine wires having different diameters. Since the size of a magnetic
fine wire affects the threshold of magnetization reversal, and
domain control, magnetic resistance, etc. in a fine wire, the
control of these properties enables application to magnetic devices
such as a magnetic sensor, a magnetoresistive element, a magnetic
recording medium, and the like.
Further objects, features and advantages of the present invention
will become apparent from the following description of the
preferred embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic plan view of a nano-structure of the present
invention;
FIG. 1B is a sectional view taken along line AA in FIG. 1A;
FIGS. 2A to D are schematic plan views respectively showing pore
arrangements of nano-structures of the present invention;
FIGS. 3A to C are drawings showing the steps of an example of a
process for preparing a nano-structure of the present
invention;
FIGS. 4A to D are schematic plan views respectively showing
patterns of pore starting points;
FIG. 5 is a schematic drawing of an anodic oxidation apparatus;
FIG. 6 is a schematic sectional view showing an example of
nano-structures of the present invention in which pores are filled
with a filler;
FIG. 7A is a graph showing a diameter distribution of pores present
in a conventional nano-structure; and
FIG. 7B is a graph showing a diameter distribution of pores present
in a nano-structure of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Construction of Nano-structure
FIG. 1 is a schematic plan view showing the construction of a
nano-structure of the present invention which has two kinds of
pores having different diameters. In FIG. 1, reference numeral 1
denotes a workpiece (aluminum); reference numeral 3, a first pore
(nano-hole); reference numeral 4, a barrier layer. Reference
numeral 5 denotes a second pore having a smaller diameter than that
of the first pores 3.
This nano-structure comprises anodic porous alumina having pores
and obtained by anodically oxidizing aluminum, the anodic porous
alumina having at least two kinds of the pores 3 and 5 having
different diameters. The pores 3 of the anodic porous alumina have
a cylindrical shape, and the pores 3 and 5 can be arranged in
substantially parallel at equal intervals.
The presence of at least two kinds of pores having different
diameters can be determined by observing a diameter distribution of
the pores. For example, as shown in FIG. 7A, the diameter
distribution of conventional nano-holes or regulated nano-holes has
only one peak. However, the diameter distribution of two kinds of
pores having different sizes in the anodic porous alumina, for
example, as shown in FIG. 2A, has two peaks, as shown in FIG. 7B.
By using this diameter distribution of pores, it can be decided
that two kinds of pores having different diameters are present.
The diameters of the pores of the anodic porous alumina can be
controlled under pore widening conditions in which the alumina is
immersed in an acid solution after anodic oxidation. In the present
invention, furthermore, at least one of the size and composition of
pore starting points can be controlled to provide at least two
kinds of pores having different diameters at predetermined
positions in the anodic porous alumina. The diameters of the
respective kinds of pores can be independently controlled. For
example, the diameter 2r of the pores 3 is several nm to several
hundreds nm, and the interval (cell size) thereof is in the range
of several nm to several hundreds nm.
The positions of the pores 3 and 5 can be controlled by controlling
the positions of the pore starting points. In some actual
applications, there is demand for forming a nano-structure having
pores at substantially equal intervals in a repetitive pattern. In
this case, the pore starting points can be formed at substantially
equal intervals in a repetitive pattern. Particularly, the
formation of pores by anodic oxidation has the tendency that pores
are formed in a triangular lattice pattern by self organization.
Therefore, the pore starting points are formed in a triangular
lattice pattern. In this case, since the voltage of anodic
oxidation and the pore interval have a correlation, the pore
starting points are also preferably set with consideration of the
interval.
The depths (lengths) of the pores 3 and 5 can be controlled by
controlling the anodic oxidation time, the thickness of Al, or the
like, and for example, the depths are in the range of 10 nm to 100
.mu.m.
The arrangement of at least two kinds of the pores 3 and 5 having
different diameters in the anodic porous alumina is not limited to
the pattern shown in FIG. 1 in which only one pore is smaller (or
larger) than pores in the periphery thereof. Examples of the
pattern include the pattern shown in FIG. 2A or B in which a region
of pores having a different diameter is partially formed, the
pattern shown in FIG. 2C in which a region of pores having a
different diameter is repeated, and the pattern shown in FIG. 2D in
which the diameter of pores is changed continuously or stepwisely
in a predetermined direction.
Furthermore, the pores in the nano-structure can be filled with a
functional material such as a metal, a semiconductor, a dielectric
material, a magnetic material, or the like. Particularly, in
filling a dielectric material, a dielectric material having a
refractive index greatly different from that of the anodically
oxidized layer is preferably used from the viewpoint of control of
the photonic band. For example, a difference between the refractive
indexes of the dielectric material used and the anodically oxidized
layer is preferably 10% or more, more preferably 50% or more.
Process for Preparing Nano-structure
The method of preparing the nano-structure of the present invention
will be described in detail below with reference to FIG. 1.
Description is made in the order of FIGS. 3A, B and C. The steps
(a) to (c) below correspond to FIGS. 3A to C, respectively.
(a) Preparation of Workpiece
The workpiece 1 is prepared. The workpiece of the present invention
has a portion mainly composed of aluminum.
Examples of the workpiece 1 include a bulk composed of Al as a main
component, and a substrate comprising a film composed of Al as a
main component. Examples of the substrate include insulator
substrates such as a quartz glass substrate, and the like;
semiconductor substrates such as a silicon substrate, a gallium
arsenic substrate, and the like; these substrates each comprising
at least one film formed thereon. The material, thickness and
mechanical strength of the substrate are not limited as long as no
problem occurs in formation of pores by anodic oxidation of a film
composed of Al as a main component. For example, by using a
substrate on which a conductive film of Ti, Nb, Pt, or Cu is
formed, the uniformity of depth of the pores can be improved. As
the method of depositing the film composed of Al as a main
component, any desired deposition method such as resistance heating
evaporation, EB deposition, sputtering, CVD, plating, or the like
can be used.
The shape of the workpiece used in the present invention is not
limited to a smooth plate shape, and a shape having a curved
surface, a shape having irregularities or steps to some extent, and
the like can be used. The shape of the workpiece is not limited as
long as no problem occurs in formation of pores by anodic
oxidation.
(b) Step of Forming Pore Starting Points
In this step, pore starting points 2 are formed at desired
positions in the portion of the workpiece which is composed of Al
as a main component. After this step, the workpiece is anodically
oxidized to form pores at the desired positions, permitting control
of the pore arrangement, interval, positions, directions, etc. of
the nano-structure. For example, it is possible to form a
nano-structure (anodic porous alumina) in which pores are regularly
arranged over the entire region of a pattern with good
linearity.
The pore starting points 2 can be formed by the method of applying
a focused ion beam 10 (FIG. 3B), the method using SPM such as AFM
and STM, the method disclosed in Japanese Patent Laid-Open No.
10-121292 in which concaves are formed by press patterning, the
method of forming concaves by etching after formation of a resist
pattern, or the like.
In this step, the composition or shape of the pore starting points
can be controlled to control the diameter of each kind of pores 3
and 5. This can realize a nano-structure in which pores having
different diameters are arrayed or arranged at controlled
positions.
In the method using a focused ion beam, irradiation conditions of
the ion beam such as the amount of irradiation, beam diameter, and
irradiation energy of the focused ion beam, can be controlled to
control the concave shape and composition of the pore starting
points. This permits control of the diameters of the final
nano-holes.
In the press patterning method, a pattern of press patterning can
be set to a desired shape to control the depth and size of concaves
of the pore starting points. This permits control of the diameters
of the final nano-holes.
In the SPM method, the pressure applied to a probe on aluminum is
controlled, or the shape of the probe is changed to control the
shape, for example, the depth or size, of concaves of the pore
starting points. Another method can also be used, in which a
voltage is applied to the probe to locally oxidize the aluminum
surface. In this method, the shape or composition of the pore
starting points can be controlled by controlling the voltage or
time.
Of these methods, the method of focused ion beam irradiation has no
need for steps requiring much labor, such as resist coating,
electron beam exposure, and resist removal, and the pore starting
points can be formed by this method within a short time. The method
also has no need to apply pressure to the workpiece, and can thus
be applied to workpieces having low mechanical strength. From these
viewpoints, the method of focused ion beam irradiation is
preferred.
The formation of the pore starting points using the focused ion
beam is described in further detail below.
The patterning positions of the aluminum surface can easily
precisely be set by using an observation function attached to a
focused ion beam processing apparatus. An example of the
observation function is to detect secondary electrons produced by
scanning the focused ion beam on a sample to obtain a scanning
image. In this observation, although the focused ion beam is
applied, a scanning image can be obtained even by using a weak
focused ion beam. Therefore, in setting patterning positions of the
workpiece, the influence of the focused ion beam irradiation can be
substantially removed. By using a focused ion beam processing
apparatus to which a scanning electron microscope, a laser
microscope, or the like having the observation function is
attached, the patterning positions of the workpiece can be set
without performing focused ion beam irradiation.
Examples of the method of moving the focused ion beam irradiation
position include the method of moving the focused ion beam, the
method of moving the workpiece, the method comprising a combination
of both methods, and the like.
Examples of ion species for the focused ion beam include liquid
metal ion sources such as Ga, Si, Ge, Cs, Nb, Cu, and the like;
field ionization gas ion sources such s O, N, He, Ar, and the like.
However, the ion species for the focused ion beam are not limited
as long as no problem occurs in formation of pores by anodic
oxidation.
As the focused ion beam, an ion beam having a diameter in the range
of about 5 to 1000 nm can be used. The focused ion beam has a
strength distribution close to a gaussian distribution in which an
irradiation size of the focused ion beam (i.e., the diameter of the
focused ion beam) at each of the pore starting points preferably
does not overlap with the irradiation area of the focused ion beam
(i.e., the diameter of the focused ion beam) at an adjacent pore
starting point.
Examples of the method of forming the pore starting points by
focused ion beam irradiation of the present invention will be
described with reference to FIGS. 4A to D. FIGS. 4A and C
respectively show examples in which the pore starting points are
formed in a substantially triangular lattice pattern, and FIGS. B
and D respectively show examples in which the pore starting points
are formed in a substantially square lattice pattern. Besides these
examples, various other examples are conceivable, and the method of
forming the pore starting points is not limited as long as no
problem occurs in pore formation by anodic oxidation.
The formation of pores by anodic oxidation has the tendency that
pores are formed in a substantially triangular lattice pattern by
self organization. Therefore, the pore starting points are
preferably formed in substantially triangular lattice pattern. This
is particularly preferred when a nano-structure having deep pores
is desired. However, with a nano-structure having shallow pores,
the above-described self organization does not occur, and it is
thus important to form the pore starting points in any desired
pattern such as a substantially square lattice pattern.
In forming the pores by anodic oxidation, the interval of the pores
can be controlled to some extent by controlling process conditions
such as the anodic oxidation voltage applied in anodic oxidation,
and thus the pore starting points are preferably formed at
intervals which are predicted from the process conditions. This is
particularly preferable for the case of a nano-structure having
deep pores. On the other hand, for a workpiece having shallow
pores, the limitation to the interval of the pore starting points
defined by the process conditions is relaxed.
An example of the ion beam irradiation method comprises irradiating
the workpiece with an ion beam in a dot form, as shown in FIGS. 4A
and B. This method comprises repeating the step of staying the
focused ion beam at an irradiation position 31 as a pore starting
point, and then moving the ion beam to a next irradiation position
31 to stay it. Where the focused ion beam is desired to be moved
even in spaces between dots, the time of movement in the spaces
between dots is set to be shorter than the staying time at each dot
position so that the influence of focus ion beam irradiation in
movement in the spaces between dots can be substantially
removed.
Another example of the ion beam irradiation method comprises
irradiating the workpiece along parallel lines 32 in two different
directions, as shown in FIGS. 4C and D. In this method, the
workpiece is significantly irradiated with the focused ion beam at
the intersections 33 of the lines as compared with the peripheral
regions, to form the pore starting points at the intersections 33
of the lines.
By using these methods, the conditions of focused ion beam
irradiation for forming the pore starting points can be controlled
so that the diameter of the final nano-holes can be controlled.
The conceivable reason why the positions significantly irradiated
with the focused ion beam become the pore starting points is that a
state different from the peripheral regions is formed at the
positions on the surface of the workpiece by ion injection or ion
etching to cause specific points in anodic oxidation. As described
above, the shape or composition of each of the pore starting points
can be controlled by controlling the amount of ion beam
irradiation, ion beam irradiation energy, beam diameter, or the
like, and thus the diameter of the final nano-holes can be
controlled.
(c) Step of Forming Pores
The workpiece is anodically oxidized to convert the portion
composed of alumina as a main component to anodic porous alumina,
to form a nano-structure. The pore starting points 2 formed in the
step (b) are reflected in the formation of the pores 3 and 5.
FIG. 5 schematically shows the anodic oxidation apparatus used in
this step. In FIG. 5, reference numeral 1 denotes a workpiece;
reference numeral 41, a constant-temperature bath; reference
numeral 42, a cathode comprising a Pt plate; reference numeral 43,
an electrolyte; reference numeral 44, a reactor; reference numeral
45, a power source for applying an anodic oxidation voltage;
reference numeral 46, an ampere meter for measuring an anodic
oxidation current. Although not shown in the drawing, a computer
for automatically controlling and measuring the voltage and
current, and the like are further provided in the apparatus.
The workpiece 1 and the cathode 42 are arranged in the electrolyte
43 kept at a constant temperature by the constant-temperature bath
41 so that a voltage is applied between the workpiece 1 and the
cathode 42 from the power source 45 to effect anodic oxidation.
The electrolyte used for anodic oxidation comprises, for example,
an oxalic acid solution, a phosphoric acid solution, a sulfuric
acid solution, a chromic acid solution, or the like. However, the
electrolyte is not limited as long as no problem occurs in
formation of pores by anodic oxidation. Forming conditions such as
the anodic oxidation voltage, temperature, etc. used can
appropriately be set according to the nano-structure formed.
Furthermore, the pores of the nano-structure can appropriately be
widened by pore widening treatment in which the nano-structure is
immersed in an acid solution (in the case of anodic porous alumina,
a phosphoric acid solution). The acid concentration, treatment
time, temperature, etc. can be controlled to obtain the
nano-structure having a desired pore diameter.
As described above, the shape or composition of each of the pore
starting points can be controlled to control the diameter and
position of each of the pores, forming the nano-structure having at
least two kinds of pores having different diameters.
In filling the pores of the nano-structure with a filler, any
desired method such as electrodeposition, vacuum melting
introduction, CVD, vacuum deposition, or the like can be used.
As described above, the present invention enables application of
the nano-structure in various forms, thereby widening the range of
application thereof.
Although the nano-structure of the present invention can be used as
a functional material, the nano-structure can also be used as a
master material, a mold, or the like for a new nano-structure.
In accordance with each of embodiments of the present invention,
for example, the following effects can be obtained.
(1) The pore starting points are formed at desired positions of a
portion containing aluminum, followed by anodic oxidation to form
pores at the desired positions. Therefore, the arrangement,
interval, positions, direction, etc. of the pores of the
nano-structure can be controlled to prepare the nano-structure
(anodic porous alumina) in which the pores are regularly arranged
over the entire region of a pattern with excellent linearity.
Particularly, at least one of the shape, size and composition of
the pore starting points can be controlled to control the pore
diameters independently. It is thus possible to realize the
nano-structure in which pores having different diameters are
arrayed or arranged at controlled positions.
(2) The pores of the nano-structure are filled with a material
having a different refractive index to permit application as an
optical material.
(3) The pores of the nano-structure are filled with a magnetic
material to permit application to magnetic fine wires, a magnetic
sensor, a magnetic recording medium, or the like.
(4) The pores of the nano-structure are filled with a luminescent
material to realize a light emitting device with a narrow emission
spectral width, a laser device with a low threshold value, or the
like.
EXAMPLES
The present invention will be described below with reference to
examples.
Example 1
In this example, pore starting points were formed by FIB.
a) Preparation of Workpiece
The surface of an Al plate having a purity of 99.99% was
mirror-processed by electric field polishing in a mixed solution of
perchloric acid and ethanol to prepare a workpiece, as shown in
FIG. 3A.
b) Step of Forming Pore Sptarting Points
The workpiece was irradiated with a focused ion beam by using a
focused ion beam processing apparatus to form pore starting points
in the workpiece, as shown in FIG. 3B. In this focused ion beam
processing apparatus, the ion species was Ga, and the acceleration
voltage was 30 kV. First, positions where the pore starting points
were formed were determined by using the secondary electron
observation function attached to the focused ion beam processing
apparatus. Next, the workpiece was irradiated with the focused ion
beam in a dot shape to form the pore starting points at intervals
of 100 nm in a substantially triangular lattice pattern, as shown
in FIG. 1. At this time, the staying time of the focused ion beam
at only one specific dot position was 10 msec, and the staying time
at each of all other dot positions was 30 msec.
c) Step of Forming Pores
The workpiece was anodically oxidized by using the anodic oxidation
apparatus shown in FIG. 5 to form pores, as shown in FIG. 1. A 0.3M
oxalic acid aqueous solution used as an acid electrolyte was
maintained at 3.degree. C. by the constant-temperature bath with an
anodic oxidation voltage of 40 V. Next, the workpiece after anodic
oxidation was immersed in a 5 wt % phosphoric acid solution for 30
minutes to widen the pores (pore widening treatment).
Evaluation (observation of structure)
As a result of observation by FE-SEM (field emission scanning
electron microscope), it was confirmed that the pore starting
points were reflected in formation of the pores. Namely, the pores
were formed to be arranged at intervals of 100 nm in a
substantially triangular lattice pattern to form a nano-structure
having the pores with high regularity.
The diameter of the pores (first pores) was about 50 nm at
positions where the amount of ion beam irradiation in formation of
the pore starting points was 30 msec, while the diameter of the
pore (second pore) was about 30 nm at the peculiar dot position
where the amount of ion beam irradiation in formation of the pore
starting points was 10 msec.
Therefore, the pore diameter could be controlled by controlling the
ion beam irradiation time (amount) as a condition for forming the
pore starting points. Particularly, the thus-formed nano-structure
had the pores regularly arranged, and containing the specific pore
5 having a smaller diameter than the peripheral pores and formed at
a controlled position, as shown in FIG. 1.
Example 2
In this example, pore starting points were formed by the press
patterning method in place of FIB.
a) Preparation of Workpiece
A workpiece was prepared according to the same procedure as Example
1.
b) Step of Forming Pore Starting Points
First, a press patterning substrate (stamper) was formed as
follows, in which two kinds of convexes were periodically
alternately arranged in a triangular lattice form.
First, a resist pattern was formed on a silicon substrate by using
an electron beam exposure apparatus, in which two kinds of
apertures having diameters of about 20 nm and 40 nm were present at
intervals of about 0.1 .mu.m in a triangular lattice form. The
apertures having diameters of about 40 nm and about 20 nm were
arranged in the same manner as the arrangement of large and small
pores shown in FIG. 2C. Then, chromium was deposited on the resist
pattern by using a deposition apparatus, and then chromium on the
resist was removed together with the resist to form two kinds of
convexes of chromium having diameters of about 25 nm and about 40
nm (the same height of 40 nm). Then, the silicon substrate was
etched with CF.sub.4 gas by a reactive dry etching process using
the chromium as a mask, and the chromium was further removed by
oxygen plasma to prepare a press patterning substrate in which two
kinds of convexes having diameters of about 25 nm and about 40 nm
and the same height of 60 nm were regularly arranged at intervals
of 0.1 .mu.m.
The press patterning substrate having the convexes formed thereon
was placed on the aluminum plate prepared in step a), and a
pressure of 3 ton/cm.sup.2 was applied by using an oil-hydraulic
press to form two kinds of pore starting points on the surface of
the aluminum plate.
c) Step of Forming Pores
Anodic oxidation and pore widening treatment were carried out by
the same method as Example 1.
Evaluation (observation of structure)
As a result of FE-SEM observation, it was confirmed that the pore
starting points were reflected in formation of the pores. Namely,
the pores were formed to be arranged at intervals of 100 nm in a
substantially triangular lattice pattern to form a nano-structure
having pores with high regularity.
In the thus-formed nano-structure, two kinds of pores 3 and 5
having diameters of 30 nm and 50 nm were periodically arranged
corresponding to the two kinds of pore starting points formed by
using the convexes having diameters of 25 nm and 40 nm, as shown in
FIG. 2C.
Example 3
In this example, pore starting points were formed in a square
lattice form by using the FIB method.
a) Preparation of Workpiece
An Al film was deposited to a thickness of 200 nm on a quartz
substrate by a resistance heating method to prepare a
workpiece.
b) Step of Forming Pore Starting Points
The Al film was irradiated with a focused ion beam in a dot shape
by using the focused ion beam processing apparatus to form pore
starting pints at intervals of 60 nm in a substantially square
lattice pattern, as shown in FIG. 4B. In the focused ion beam
processing apparatus, the ion species was Ga, the acceleration
voltage was 15 kV, the ion beam diameter was 30 nm, the ion current
was 2 pA, and the staying time of the focused ion beam was 30 msec.
However, the acceleration voltage at dots on only one line of the
square lattice was 30 kV.
c) Step of Forming Pores
Anodic oxidation and pore widening treatment were carried out by
the same method as Example 1 in which a 0.3M sulfuric acid aqueous
solution was used as the electrolyte, and the solution was kept at
3.degree. C. by the constant-temperature bath with an anodic
oxidation voltage of 25 V.
Evaluation (observation of structure)
As a result of FE-SEM observation, it was confirmed that the pore
starting points were reflected in formation of the pores. Namely,
the pores were formed to be arranged at intervals of 60 nm in a
substantially square lattice pattern to form a nano-structure
having pores with high regularity.
The diameter of the pores was about 25 nm at positions where an ion
acceleration voltage of 15 kV was applied in formation of the pore
starting points, while the diameter of the pores was about 45 nm at
positions where an ion acceleration voltage of 30 kV was
applied.
Therefore, the ion acceleration voltage as a condition for forming
the pore starting points was controlled to form the nano-structure
in which the pores were regularly arranged, and the pores arranged
in a line had a larger diameter than that of the peripheral
pores.
Example 4
In this example, three kinds of pore starting points were formed by
the FIB method under different irradiation conditions.
a) Preparation of Workpiece
As shown in FIG. 6, a Nb film was deposited as a conductive film 15
having a thickness of 100 nm on a Si substrate 16 by the
electron-beam deposition method, and then an Al film 12 of 500 nm
was deposited by the sputtering method to prepare a workpiece
1.
b) Step of Forming Pore Starting Points
The Al film was linearly irradiated with a focused ion beam by
using the focused ion beam processing apparatus so that
substantially parallel lines were formed at intervals of 100 nm.
The ion current was controlled to 1 pA and 2 pA for each line.
Furthermore, the Al film was linearly irradiated with the focused
ion beam in the direction at 60.degree. with the previous lines so
that the ion current was 1 pA and 2 pA for each line. The ion
species of the focused ion beam was Ga, the acceleration voltage
was 30 kV, and the diameter of the ion beam was 30 nm. The scan
speed and the number of scans were controlled so that the total
staying time of the focused ion beam at each of the intersections
of the lines was 20 msec. As a result, three kinds of pore starting
points respectively having different total amounts of ion
irradiation of 40 f Coulomb, 60 f Coulomb, and 80 f Coulomb were
formed in a triangular lattice pattern.
c) Step of Forming Pores
Anodic oxidation and pore widening treatment were carried out by
the same method as Example 1 in which a 0.3M oxalic acid aqueous
solution was used as the electrolyte, and the solution was kept at
3.degree. C. by the constant-temperature bath with an anodic
oxidation voltage of 45 V.
The anodic oxidation current was monitored to confirm by a decrease
in the anodic oxidation current that aluminum was converted to
alumina over the total thickness. Then, anodic oxidation was
finished.
Evaluation (observation of structure)
As a result of FE-SEM (field emission scanning electron microscope)
observation, it was confirmed that the pore starting points were
reflected in formation of the pores. Namely, the pores were formed
to be arranged at intervals of 115 nm in a substantially triangular
lattice pattern to form a nano-structure having pores with high
regularity.
The diameters of the pores were about 25 nm, about 40 nm, and about
50 nm corresponding to the three kinds of pore starting points
where the amounts of ion irradiation in formation of the pore
starting points were 40 f Coulomb, 60 f Coulomb, and 80 f Coulomb,
respectively.
Therefore, the ion current in formation of the pore starting points
was controlled to form the nano-structure in which three kinds of
pores were regularly arranged.
Example 5
In this example, pores were filled with a magnetic material to
prepare a nano-structure.
a) Preparation of Workpiece
A workpiece 1 was prepared by the same method as Example 4.
b) Step of Forming Pore Starting Points
The Al film was linearly irradiated with a focused ion beam by
using the focused ion beam processing apparatus so that
substantially parallel lines were formed at intervals of 100 nm.
The ion current was controlled to 1 pA and 2 pA for each line.
Furthermore, the Al film was linearly irradiated with the focused
ion beam in the direction at 60.degree. with the previous lines so
that the ion current was 2 pA for all lines. The ion species of the
focused ion beam was Ga, the acceleration voltage was 30 kV, and
the diameter of the ion beam was 30 nm. The scan speed and the
number of scans were controlled so that the total staying time of
the focused ion beam at each of the intersections of the lines was
30 msec. As a result, two kinds of pore starting points were formed
at the line intersections where the total amounts of ion
irradiation were 60 f Coulomb and 90 f Coulomb.
c) Step of Forming Pores
Anodic oxidation and pore widening treatment were carried out by
the same method as Example 1 in which a 0.3M oxalic acid aqueous
solution was used as the electrolyte, and the solution was kept at
3.degree. C. by the constant-temperature bath with an anodic
oxidation voltage of 45 V.
The anodic oxidation current was monitored to confirm by a decrease
in the anodic oxidation current that aluminum was converted to
alumina over the total thickness. Then, voltage application was
finished.
d) Step of Filling Pores With a Metal
Next, the pores were filled with a filler 6 by electrodeposition of
Ni metal. In filling the pores with Ni, Ni was precipitated in the
nano-holes by electrodeposition in which the workpiece was immersed
in an electrolyte containing 0.14M NiSO.sub.4 and 0.5 M H.sub.3
BO.sub.3 together with a Ni counter electrode to precipitate Ni in
the nano-holes.
Evaluation (observation of structure)
As a result of FE-SEM observation, the pores were filled with Ni to
form magnetic nano-wires composed of Ni and having diameters of 30
nm and 50 nm according to the sizes of the large and small pores,
as shown in FIG. 6. Although FIG. 6 shows a structure in which at
the bottom of each of the pores, the filler directly contacts the
lower conductive film 15, the structure at the bottom of each pore
is not limited to this. A structure in which an insulating barrier
layer 4 is formed at the bottom of each pore, a structure in which
the barrier layer 4 contains a conductive pass (not shown), or the
like can be used according to the material of the conductive film
15, and anodic oxidation conditions. This example has a structure
in which the barrier layer is formed at the bottom of each pore,
and contains a conductive pass, filler Ni (6) being electrically
connected to the lower conductive film 15 through the conductive
pass.
As a result of measurement of magnetic susceptibility, a two-step
magnetization curve was observed. This is possibly due to the fact
that the presence of two kinds of magnetic materials having
different wire diameters makes the magnetization curve stepwise due
to a difference in anisotropic energy. By using the property that
magnetization is reversed in a magnetic field depending upon the
diameters of fine wires, application to various magnetic devices
can be expected.
Example 6
In this example, pores were filled with a layered film comprising a
magnetic material and a nonmagnetic material to form a
nano-structure.
In this example, alumina nano-holes were formed by the same anodic
oxidation as Example 5, and then filled with a lamination of
metals. The lower conductive film 15 (FIG. 6) was composed of
Pt.
After the pores were formed, a sample was immersed in an
electrolyte comprising 0.5M of cobalt sulfate and 0.001M copper
sulfate together with a platinum counter electrode, with voltages
0.2 V and 0.9 V alternately applied for 1 second and 15 seconds,
respectively, to grow Co and Cu layers at the bottom of each
nano-hole. In this step, only Cu as a low-concentration ion was
electro-deposited with a voltage of 0.2 V applied, and a high
concentration of Co was mainly electro-deposited with a voltage of
0.9 V applied to form a layered film. This example has a structure
in which the filler at the bottom of each pore directly contacts
the lower conductive film 15, as shown in FIG. 6, so that the lower
conductive film 15 is electrically connected to the layered film 6
of Co and Cu.
An electrode was attached to the upper portion of the
nano-structure of the present invention to examine the magnetic
field dependency of resistance between the upper portion of the
filler and the lower layer. As a result, stepwise negative magnetic
resistance was observed. This is possibly due to the fact that the
layered film deposited in each of the pores exhibits a GMR effect.
It is thus found that the nano-structure of this example can be
used for a magnetic sensor.
Example 7
a) A workpiece was Prepared According to the Same Procedure as
Example 1.
b) Step of Forming Pore Starting Points
Pore starting points were formed by the same method as Example 5.
The interval of the pore starting points was 200 nm.
c) Step of Forming Pores
Anodic oxidation and pore widening treatment were carried out by
the same method as Example 1 in which a 0.3M phosphoric acid
aqueous solution was used as the electrolyte, and the solution was
kept at 3.degree. C. by the constant-temperature bath with an
anodic oxidation voltage of 80 V.
As a result of measurement of a transmission spectrum of anodic
porous alumina isolated by dissolving with silver chloride, a
decrease in transmittance was observed at a wavelength from 500 nm
to 600 nm. This indicated that the anodic porous alumina exhibit
properties as a photonic crystal. It was thus found that the
nano-structure of the present invention can be used for an optical
device.
Example 8
In this example, regular anodic porous alumina was formed, and
pores were filled with an oxide.
Anodic porous alumina was formed on Pt by the same method as
Example 6. However, the interval of pore starting points was 160
nm, a 5 wt % phosphoric acid solution was used as an electrolyte
for anodic oxidation, and the voltage was set to 65 V.
After the step of forming pores, a sample as immersed in a 0.1M
zinc nitride aqueous solution kept at 60.degree. C. together with a
platinum counter electrode, and a voltage of 0.8 V based on a
Ag/AgCl standard electrode was applied to grow ZnO crystals in the
pores.
As a result of FE-SEM observation, it was found that the pores were
regularly arranged, and ZnO was grown in the pores.
As a comparative example, ZnO was deposited on Pt without
nano-holes under the same conditions.
As a result of irradiation of the nano-structure of the present
invention with He-Cd laser (wavelength 325 nm), strong light
emission with a narrow spectral width was observed at a wavelength
of near 390 nm, as compared with the comparative example.
The results of this example reveal that the pores of anodic porous
alumina can be filled with a luminescent material (ZnO). It was
also found that filling pores with a luminescent material permits
application to optical devices.
While the present invention has been described with reference to
what are presently considered to be the preferred embodiments, it
is to be understood that the invention is not limited to the
disclosed embodiments. On the contrary, the invention is intended
to cover various modifications and equivalent arrangements included
within the spirit and scope of the appended claims. The scope of
the following claims is to be accorded the broadest interpretation
so as to encompass all such modifications and equivalent structures
and functions.
* * * * *